1. Field of the Invention
This invention relates to a novel method of producing hydrates of sodium dichlorocyanurate, including hydrates of sodium dichloroisocyanurate. More particularly, this invention relates to the direct conversion of wet sodium dichlorocyanurate to the hydrated form, i.e., the monohydrate, the dihydrate or approximations thereof.
2. Description of the Prior Art
The alkali metal dichlorocyanurates, sometimes termed alkali metal dichloroisocyanurates, are well known materials which are widely used as a source of available chlorine in sanitizing and bleaching applications. Of these, the sodium salt is the most widely used. It is known to exist in the anhydrous form; as the monohydrate (approximately 7.6% combined water of hydration by weight); and as the dihydrate (14.1% combined water). See, for example, U.S. Pat. No. 3,035,056 issued May 15, 1962; U.S. Pat. No. 3,035,057 issued May 15, 1962; and U.S. Pat. No. 3,294,797 issued Dec. 27, 1966.
It is known to prepare sodium dichlorocyanurate by several different methods. One typical method is to react dichlorocyanuric acid with sodium hydroxide under controlled conditions, thereby producing a slurry which contains sodium dichlorocyanurate dihydrate. The product is then filtered from the slurry and dried to the anhydrous form. Another method of preparing sodium dichlorocyanurate involves the direct chlorination of trisodium isocyanurate in an aqueous medium under controlled conditions, followed by filtration of the product and drying to the anhydrous form. Still another method involves bringing together and reacting trichlorocyanuric acid and trisodium cyanurate in an aqueous medium under controlled conditions to produce an aqueous slurry of sodium dichlorocyanurate dihydrate followed by filtration and drying steps to produce the anhydrous form.
Typically, the sodium dichlorocyanurate dihydrate is filtered or centrifuged to produce a wet cake of sodium dichlorocyanurate dihydrate. The latter material can then be dried by one of several methods known to those skilled in the art to produce a free flowing particulate sodium dichlorocyanurate dihydrate. It is known to then convert the dihydrate form to the anhydrous form by heating it at about 105.degree.C. for about 8 hours in a hot air circulating oven as described in U.S. Pat. No. 3,035,056.
It is well known that the alkali metal dichlorocyanurates, as well as certain other chlorinated dry bleach compounds, when exposed to a flame, spark or other high temperature source, can begin to burn and continue burning after the initial heat source has been removed until all of the material is consumed. This phenomenon is referred to as self-sustaining and self-propagating decomposition. It is further known that the presence of combined water of hydration lessens the tendency toward self-propagating decomposition. Thus, sodium dichlorocyanurate monohydrate is less susceptible to self-propagating decomposition than anhydrous sodium dichlorocyanurate. Sodium dichlorocyanurate dihydrate is in turn even less susceptible than the monohydrate. The presence of bound water of hydration has also been found to relieve dust problems associated with the anhydrous product.
Thus, the advantageous properties of sodium dichlorocyanurate monohydrate and dihydrate make those products attractive for many bleaching, sterilizing, sanitizing and disinfecting applications, notwithstanding an initial loss of available chlorine as compared to the anhydrous product. For example, anhydrous sodium dichlorocyanurate usually contains initially about 63 percent available chlorine as compared to about 59 percent available chlorine in the monohydrate and about 56 percent available chlorine in the dihydrate.
Lack of commercial success for the monohydrate of sodium dichlorocyanurate can be ascribed in part to the difficulty in manufacture. For example, it has been found that the use of oven drying or carrier drying techniques usually result in either the loss of both molecules of bound water or the loss of neither molecule of bound water. Furthermore, production of the monohydrate form of sodium dichlorocyanurate has heretofore involved the use of vacuum drying equipment. In U.S. Pat. No. 3,035,056 there is found an illustration of the dehydration of sodium dichlorocyanurate dihydrate to the monohydrate by "drying at 70.degree.C. under vacuum for about 3 hours."